The fluidized catalytic cracking (FCC) of heavy hydrocarbons to produce lower boiling hydrocarbon products such as gasoline is well known in the art. FCC processes have been around since the 1940's. Typically, an FCC unit or process includes a riser reactor, a catalyst separator and stripper, and a regenerator. A FCC feedstock is introduced into the riser reactor wherein it is contacted with hot FCC catalyst from the regenerator. The mixture of the feedstock and FCC catalyst passes through the riser reactor and into the catalyst separator wherein the cracked product is separated from the FCC catalyst. The separated cracked product passes from the catalyst separator to a downstream separation system and the separated catalyst passes to the regenerator where the coke deposited on the FCC catalyst during the cracking reaction is burned off the catalyst to provide a regenerated catalyst. The resulting regenerated catalyst is used as the aforementioned hot FCC catalyst and is mixed with the FCC feedstock that is introduced into the riser reactor.
U.S. Pat. No. 3,928,172 discloses an arrangement of reactors that provides for the gasoline product of gas oil cracking to be re-cracked in a dense fluid catalyst bed reaction zone with freshly regenerated catalyst. The used catalyst from the gasoline re-cracking reactor is used for the gas oil cracking in a riser cracking zone; however, there is no teaching that a portion of the freshly regenerated catalyst may bypass the gasoline re-cracking reactor to be used in combination with the used catalyst from gasoline re-cracking reactor in the riser cracking zone. This patent focuses on the production of gasoline that satisfies certain quality specifications, and, thus, it is not directed to a process for converting gas oil to preferentially yield middle distillate and lower olefin products. Therefore, the patent fails to recognize numerous various means by which higher lower olefin and middle distillate yields may be achieved in the above-described process arrangement involving the re-cracking of heavy gasoline in the boiling range of 260° F. to 380° F.
U.S. Pat. No. 5,372,704 discloses a process for the cracking and re-cracking of liquid hydrocarbons. Feed to the re-cracking reactor may be FCC heavy naphtha. The re-cracking reactor uses spent catalyst from the FCC reactor with up to 100% of the catalyst used in the re-cracker reactor being spent FCC catalyst. The re-cracking catalyst activity is controlled by mixing hot regenerated FCC catalyst with the spent FCC catalyst. The re-cracking catalyst mixture includes from 50 to 95 wt % spent FCC catalyst and from 5 to 50 wt % regenerated FCC catalyst. Spent re-cracking catalyst may be used in the FCC riser reactor along with hot regenerated FCC catalyst. This patent focuses on improving the quality of FCC gasoline and generating higher yields of gasoline from refinery streams, and it is not directed to a process for converting gas oil to preferentially yield middle distillate and lower olefin products. Therefore, the patent fails to recognize numerous various means by which higher lower olefin and middle distillate yields may be achieved in its process.
As noted above, many FCC processes and systems are designed so as to provide for a high conversion of the FCC feedstock to products having boiling temperatures in the gasoline boiling range. There are situations, however, when it is desirable to provide for the high conversion of the FCC feedstock to middle distillate boiling range products, as opposed to gasoline boiling range products, and to lower olefins. However, making lower olefins requires high severity and high reaction temperature reaction conditions. These conditions normally result in low middle distillate product yield and poor middle distillate product quality. It is therefore very difficult in the conventional cracking of hydrocarbons to provide simultaneously for both a high yield of lower olefins and a high yield of middle distillate products.